1. Field of the Invention
The present invention related to methods and apparatus for high resolution imaging of samples using a superconductive quantum interference device (SQUID).
2. Background Art
This section describes the background of the disclosed embodiments of the present invention. There is no intention, either expressed or implied, that the background art discussed in this section legally constitutes prior art.
There have been a variety of different types and kinds of devices for imaging samples. One example of apparatus and methods used for high resolution imaging employs superconducting quantum interference devices (SQUIDs). For example, reference may be made to the following articles, each of which is incorporated herein by reference:
J. P. Wikswo, Jr., High-Resolution Magnetic Imaging: Cellular Action Currents and Other Applications, edited by H. Weinstock (Kluwer Academic, The Netherlands, 1996), Vol. 329, pp. 307–360.
I. M. Thomas, S. M. Freake, S. J. Swithenby, and J. P. Wikswo, Jr., Phys. Med. Biol. 38, 1311 (1993).
I. M. Thomas, T. C. Moyer, and J. P. Wikswo, Jr., Trans., Am. Geophys. Union 72, 138 (1991).
S. Chatraphorn, E. F. Fleet, and F. C. Wellstood, Bull. Am. Phys. Soc. 44, 1554 (1999).
J. R. Kirtley, C. C. Tsuci, M. Rupp, J. Z. Sun, L. S. Yu-Jabnes, A. Gupta, M. B. Ketchen, K. A. Moler, and M. Bhushan, Phys, Rev. Letr. 76, 1336 (1996).
Y. R. Chemla, H. L. Grossman, T. S. Lee, J. Clarke, M. Adamkiewicz, and B. B. Buchanan, Biophys. J. 76, 1336 (1996).
H. Weinstock, IEEE Trans. Magn. 27, 3231 (1991).
A. Cochran, G. B. Donaldson, C. Carr, D. M. McKirdy, M. E. Walker, and U. Klein, Rev. Prog. Quant. Nondestr. Eval. 15, 1151 (1996).
A. Abedi, J. Fellenstein, A. J. Lucas, and J. P. Wikswo, Jr., Rev. Sci. Instrum. 70, 4640 (1999).
T. Varpula and H. Seppa, Rev. Sci. Instrum, 64, 1593 (1993).
D. S. Buchanan, D. B. Crum, D. Cox, and J. P. Wikswo, Jr., in Micro-SQUID: A Close-Space Four Channel Magnetometer, edited by S. J. Williamson, M. Hoke, G. Stroink, and M. Ktani (Plenum, N.Y., 1990), pp. 677–679.
J. R. Kirtley, M. B. Ketchen, K. G. Stawiasz, J. Z. Sun, W. J. Gallagher, S. H. Blanton, and S. J. Wind, Appl. Phys. Lett. 66, 1138 (1995).
R. C. Black, A. Mathai, F. C. Wellstood, E. Dantsker, A. H. Miklich, D. T. Nemeth, J. J. Kingston, and J. Clarke, Appl. Phys. Lett. 62, 2128 (1993).
T. S. Loc, E. Dantsker, and J. Clarke, Rev. Sci. Instrum. 67, 4208 (1996).
C. D. Tesche and J. Clarke, J. Low Temp. Phys. 29, 301 (1977).
E. Dantsker, S. Tanaka, P. A. Nilsson, R. Kleiner, and J. Clarke, IEEE Trans. Appl. Supercond. 7, 2772 (1997).
Y. S. Touloukian and E. H. Buyco, Thermophysical Properties of Matter (IFI/Plenum, N.Y., 1970), Vol. 5.
J. R. Clem, IEEE Trans. Magn. 23, 1093 (1987).
J. Clarke, in SQUID Fundamentals, edited by H. Weinstock (Kluwer Academic, The Netherlands, 1996), Vol. 329, pp.1–62.
J. R. Kirtley and J. P. Wikswo, Jr., Annu. Rev. Mater. Sci. 29, 117 (1999).
B. J. Roth and J. P. Wikswo, Jr., Bull. Am. Phys. Soc. 61, 2439 (1990).
S. Tan, B. J. Roth, and J. P. Wikswo, Jr., Bull. Am. Phys. Soc. 34, 1301 (1989).
F. C. Wellstood, Y. Gim, A. Amar, R. C. Black, and A. Mathai, IEEE Trans. Appl. Supercond. 7, 3134 (1997).
B. P. Weiss, J. L. Kirschvink, F. J. Baudenbacher, H. Vali, N. T. Peters, F. A. Macdonald, and J. P. Wikswo, Science 290, 791 (2000).
B. P. Weiss, H. Vali, F. J. Baudenbacher, J. L. Kirschvink, S. T. Stewart, and D. L. Shuster, Earth and Planetary Science Letters 201, 449 (2002).
T. S. Lee, Y. R. Chemla, E. Dantsker, and J. Clarke, IEEE Trans. Appl. Supercond. 7, 3147 (1997).
M. B. Ketchen, D. D. Awschalom, W. J. Gallagher, A. W. Kleinsasser, R. Sandstrom, J. R. Rozen, and B. Bumble, IEEE Trans. Magn. 25, 1212 (1989).
D. J. Stanton, J. P. Wikswo Jr., Magnetic Inverse Method for Determination of Anisotropic Electrical Conductivities in a Two-Dimensional Cardiac Bidomin, IOS Press, Amsterdam, The Netherlands, 1995.
J. P. Wikswo Jr., in: R. W. Fast (Ed.), High-Resolution Measurements of Biomagnetic Fields, vol. 33, Plenum, N.Y., 1988, pp.107–116.
T. S. Lee, E. Dantsker, J. Clarke, Rev. Scientific Instrum. 67 (12) (1996) 4208.
C. S. Henriquez, Crit. Rev. Biomed. Engng. 2 (1) (1993) 1.
B. J. Roth, J. P. Wikswo Jr., IEEE Trans. Biomed. Engng. 33 (4) (1986) 467.
B. J. Roth, J. P. Wikswo Jr., IEEE Trans. Biomed. Engng. 41 (3) (1994) 232.
S.-F. Lin, J. P. Wikswo Jr., J. Biomed. Opt. 4 (2) (1999) 200.
B. J. Roth, N. G. Sepulveda, J. P. Wikswo Jr., J. Appl. Phys. 65 (1) (1989) 361.
N. G. Sepulveda, B. J. Roth, J. P. Wikswo Jr., Biophys. J. 55 (5) (1989) 987.
J. P. Barach, J. P. Wikswo Jr., IEEE Trans. Biomed. Engng. 41 (10) (1994) 969.
In order to provide high resolution imaging employing SQUID apparatus, it is important to have close tolerance spacing between the sensor being or using a SQUID device and the sample under investigation. In this regard, reference may be made to U.S. Pat. Nos. 5,491,411 and 5,894,220, which are incorporated herein by reference. In the patented systems, the entire SQUID device must be moved relative to a window separating the SQUID sensor and the sample. Such an adjustment apparatus does not lend itself to extremely close tolerance spacing between the SQUID sensor and the sample.
Another problem associated with the patented systems is that with such close tolerance spacing, it is important to maintain the window of the SQUID and the platform for supporting the room-temperature sample in a parallel relationship. Also, the patents do not disclose the manner in which the sensor is connected or matched to the system to provide the desired high resolution imaging.
Thus, it would be highly desirable to provide a new and improved high resolution imaging system.
Superconducting quantum interference device (SQUID) magnetometers have surpassed energy sensitivity and have been used to provide images of the magnetic field distributions associated with nerve and muscle action currents (see the Wikswo, et al. article), development currents in the chick embryo (see first-mentioned Thomas, et al. article), remnant magnetization in geological thin sections (see the second-mentioned Thomas, et al. article) currents in integrated circuits (see the Chatraphorn, et al., article), trapped flux in superconductors (see the Kirtley, et al. article), motion of magnetotatic bacteria (see the Chemla, et al. article), cracks and defects in metals (see the Weinstock, et al. and the Cochran, et al. articles), ongoing corrosion activity (see the Abedi, et al. article), and Johnson noise (see the Varpula, et al. article). Scanning SQUID microscopes are limited by the often-conflicted demands for high field sensitivity, which requires large sensing volumes and low noise SQUIDs, and high spatial resolution, requiring small sensing volumes, in close proximity to the sample.
One of the often-stated advantages of high-temperature superconductivity (HTS) over the more advanced low-temperature superconductivity (LTS) is that the higher operating temperature, typically around 77 K, allows HTS SQUIDs to be placed in closer proximity to room temperature samples than LTS ones with their lower operating temperature (4–10 K). In this regard, scanning SQUID magnetometers have achieved separations between the 4 K sensor and a room-temperature sample of only 1 to 2 mm for some applications, with comparable spatial resolution (see the Crum, et al. article). The most notable successes for LTS SQUID microscopes in terms of spatial resolution and sample to sensor spacing have had the SQUID apparatus and the sample both cooled to cryogenic temperatures and in a common vacuum space (see the Buchanan, et al. article). In contrast, HTS SQUID microscopes have achieved 15–50 micrometer separations typically and 50 micrometer spatial resolution (see the Black, et al. and the first Lee, et al. articles). However, because of their higher operating temperature, HTS SQUIDS have intrinsic noise levels at 1 kHz that are a factor of 4 to 5 higher than that of LTS SQUIDs (see the Tesche, et al. article). Unfortunately, HTS SQUIDs may suffer from excess 1/f noise associated with flux motion in the bulk superconductor and critical current fluctuations in the Josephson junctions (see the Dantsker, et al. article). Hence HTS SQUIDs have not yet always provided the combined low-frequency sensitivity and high spatial resolution required for magnetic imaging of many applications, such as bioelectric currents in living tissue and weak remnant magnetization in geological specimens.
Important technological features for some applications for both LTS and HTS SQUID microscopes is to locate the cryogenic SQUID apparatus in vacuum behind a thin room-temperature window (see, for example, the Buchanan, et al. and the Black, et al. articles). The SQUID apparatus is cooled by a high thermal conductivity link to a cryogen reservoir in the same vacuum space. While the vaporization enthalpy of liquid nitrogen is a factor of approximately 34 greater than that of liquid helium, for HTS microscopes this advantage may be only partly offset by a lower thermal conductivity of a typical thermal-link materials at 77 K as compared to 4 K. The thermal conductivity ratio for copper increases by a factor of 27 whereas that for sapphire decreases by a factor of 10 [κCu(4 K)/κCu(77 K)=16200/600=27 compared to κsapphire(4 K)/κsapphire(77 K)=110/1100=0.1]. The radiative heat load delivered to the SQUID from the room temperature window and surrounding hardware is independent of the window-to-SQUID separation and is essentially the same for HTS and LTS microscopes (within about 2% for some applications). These numbers suggest that the thermal design for an LTS microscope is only slightly more challenging than for an HTS SQUID microscope. This, coupled with the fact that the lower intrinsic and I/f noise of LTS SQUIDs can provide better sensitivity especially in the low frequency range (<1 Hz) than HTS SQUIDs, provides strong motivation for developing a high-resolution LTS SQUID microscope.
Electric currents play a key role in a wide range of biological phenomena. One of the most important findings from our experimental and theoretical studies is that the ability to measure cellular action currents directly, without assumptions regarding tissue conductivities or anisotropies, can provide new and valuable insights into a number of areas. See, for example, the first mentioned Wikswo, et al article. These include the interplay between tissue properties, electric fields and currents, and the propagation of electrical activity in multicellular systems, especially those with anisotropies in their electrical conductivities. There are a number of poorly understood phenomena in cardiac electrophysiology resulting from unequal tissue anisotropies and heterogeneieties. Theoretical analysis indicates that magnetic discrimination between models describing these phenomena is most accurate at spatial frequencies above 1 mm− (See the forementioned Staton, et al article.) Furthermore, bioelectric sources of magnetic fields are often distributed over a region of this scale. In order to measure these fields, for some applications it may be important to have a field sensitivity on the order of a few 100 fT Hz−1/2 at frequencies from about 1 Hz to 1 kHz (see the second Wikswo, et al article). This may be achieved by low temperature superconducting quantum interference device (SQUID) magnetometers with the required spatial resolution. In order to obtain high spatial resolutions a 4.2 K sensor must be placed in close proximity to the room temperature sample, typically at distances comparable to the spatial resolution. As hereinafter described in greater detail, a scanning SQUID microscope (SSM) is optimized for imaging biomagnetic fields and present initial measurements of magnetic fields associated with current injection and the propagation of action currents in cardiac tissue.